This study will quantitate the effects of a decrease in deformability of the red cell on tissue metabolism as measured by biochemical parameters. Deformability will be measured by the rapidity with which the red cell passes through a specific polycarbonate membrane with an average four micron pore size. Washed rat erythrocytes will be incubated at 37 degrees C for specific time periods to give degrees of red cell non-deformability. The "non-deformable" red cells will be injected into an anesthetized rat, replacing most of the recipient's red cells by exchange transfusion. Flow through the common carotid artery will be monitored. Experiments will be carried out during hypotension or hypoxia to mimic stressful clinical situations. The rat brain will be frozen and removed for subsequent analysis of high energy phosphates and certain putative neurotransmitters. Changes in the tissue biochemical parameters will be correlated with the degree of deformability of the perfused red cells under the varying conditions of hypoxia and hypotension.